The present invention relates to an anti-lock controlled brake system for an automotive vehicle, including a braking pressure generator, an auxiliary-pressure source and wheel sensors and electronic circuits for the determination of the rotational behavior of the vehicle wheels and for the generation of electric braking pressure control signals which provide for control of pressure-fluid inlelt valves and pressure-fluid outlet valves in the brake system pressure-fluid lines.
A known combined traction control and anti-lock control apparatus is disclosed in German published patent application No. 20 49 262 wherein the anti-lock system is designed as a four-channel system, there being provision of rotational speed sensors for monitoring the state of motion of the driven and the non-driven vehicle wheels. The sensors deliver in each case a voltage output signal that is proportional to the wheel circumferential speed.An inlet valve is provided for each individual wheel brake and provides for metering the braking pressure generated during actuation of the master brake cylinder into the respective wheel brake cylinder. An outlet valve is also provided through which brake fluid can be discharged from the wheel brake cylinder back into the supply reservoir of the brake system. The inlet valves and outlet valves are designed as two-way/two-position solenoid valves, with the initial postion of the inlet valve being normally open whereby the associated outlet pressure chamber of the master brake cylinder communicates with the respective wheel brake cylinder. The initial position of the outlet valve is normally closed whereby the wheel brake cylinder is noramlly shut off towards the supply reservoir. The inlet valves and outlet valves provided within the anti-lock system are controlable to assume their closed or opened position in the combination and sequence according to the anti-lock control through output control signals from an electronic control unit of the anti-lock system. The control unit processes the output signals from the wheel rotational speed sensors. For example, if a braking pressure decrease becomes necessary in one of the wheel brakes, the inlet valve associated with that wheel brake is actuated to assume its closed position and the outlet valve is actuated to assume its opened position.
This known brake slip control apparatus bears the disadvantage that the engine's residual torque, that is, the torque of the vehicle engine which acts on the driving-axle wheels even after controlled reduction of engine speed, counteracts control of the flux of force to the drive wheels.
It is known that a vehicle may reach an unstable driving condition if it rolls without being driven on a road surface with a particularly low frictional value. This occurs because the residual torque of the engine slows down the driving-axle wheels so far that the wheels get into the slip range. Conventional anti-lock control apparatuses regulate the brakes, for example on icy roads, to such extent that scarcely any braking effect is produced by the brake. However, conventional anti-lock control apparatuses aare unable to suppress the detrimental effect of the engine's residual torque during anti-lock control.